From: 011netservice@gmail.com
Date: 2024-03-19
Subject: SslStreamClass.txt

#### 摘要
2024-05-14
以 //**** 在下方區塊 SslStream Class 中摘要說明.


#### SslStream Class
2024-03-19
https://learn.microsoft.com/en-us/dotnet/api/system.net.security.sslstream?view=net-8.0&redirectedfrom=MSDN

Provides a stream used for client-server communication that uses the Secure Socket Layer (SSL) security protocol to authenticate the server and optionally the client.
public class SslStream : System.Net.Security.AuthenticatedStream

**** TcpListener //**** TcpListener 範例:
The following code example demonstrates creating an TcpListener that uses the SslStream class to communicate with clients.
using System;
using System.Collections;
using System.Net;
using System.Net.Sockets;
using System.Net.Security;
using System.Security.Authentication;
using System.Text;
using System.Security.Cryptography.X509Certificates;
using System.IO;

namespace Examples.System.Net
{
    public sealed class SslTcpServer
    {
        static X509Certificate serverCertificate = null;
        // The certificate parameter specifies the name of the file
        // containing the machine certificate.
        public static void RunServer(string certificate)
        {
			//**** 取得憑證檔案: 在 listener.Start() 啟動之前取得 serverCertificate 憑證檔案
            serverCertificate = X509Certificate.CreateFromCertFile(certificate);
            // Create a TCP/IP (IPv4) socket and listen for incoming connections.
            TcpListener listener = new TcpListener(IPAddress.Any, 5000);
            listener.Start();
            while (true)
            {
                Console.WriteLine("Waiting for a client to connect...");
                // Application blocks while waiting for an incoming connection.
                // Type CNTL-C to terminate the server.
                TcpClient client = listener.AcceptTcpClient();
                ProcessClient(client);
            }
        }
        static void ProcessClient (TcpClient client)
        {
            // A client has connected. Create the SslStream using the client's network stream.
			// 建立與 client 通訊的 sslStream
            SslStream sslStream = new SslStream(client.GetStream(), false);
            // Authenticate the server but don't require the client to authenticate.
			// 對伺服器進行身份驗證，但不要求客戶端進行身份驗證。
            try
            {
                sslStream.AuthenticateAsServer(serverCertificate, clientCertificateRequired: false, checkCertificateRevocation: true);

                // Display the properties and settings for the authenticated stream.
                DisplaySecurityLevel(sslStream);
                DisplaySecurityServices(sslStream);
                DisplayCertificateInformation(sslStream);
                DisplayStreamProperties(sslStream);

                // Set timeouts for the read and write to 5 seconds.
                sslStream.ReadTimeout = 5000;
                sslStream.WriteTimeout = 5000;
                // Read a message from the client.
                Console.WriteLine("Waiting for client message...");
                string messageData = ReadMessage(sslStream);
                Console.WriteLine("Received: {0}", messageData);

                // Write a message to the client.
                byte[] message = Encoding.UTF8.GetBytes("Hello from the server.<EOF>");
                Console.WriteLine("Sending hello message.");
                sslStream.Write(message);
            }
            catch (AuthenticationException e)
            {
                Console.WriteLine("Exception: {0}", e.Message);
                if (e.InnerException != null)
                {
                    Console.WriteLine("Inner exception: {0}", e.InnerException.Message);
                }
                Console.WriteLine ("Authentication failed - closing the connection.");
                sslStream.Close();
                client.Close();
                return;
            }
            finally
            {
                // The client stream will be closed with the sslStream
                // because we specified this behavior when creating
                // the sslStream.
                sslStream.Close();
                client.Close();
            }
        }
        static string ReadMessage(SslStream sslStream)
        {
            // Read the  message sent by the client.
            // The client signals the end of the message using the
            // "<EOF>" marker.
            byte [] buffer = new byte[2048];
            StringBuilder messageData = new StringBuilder();
            int bytes = -1;
            do
            {
                // Read the client's test message.
                bytes = sslStream.Read(buffer, 0, buffer.Length);

                // Use Decoder class to convert from bytes to UTF8
                // in case a character spans two buffers.
                Decoder decoder = Encoding.UTF8.GetDecoder();
                char[] chars = new char[decoder.GetCharCount(buffer,0,bytes)];
                decoder.GetChars(buffer, 0, bytes, chars,0);
                messageData.Append (chars);
                // Check for EOF or an empty message.
                if (messageData.ToString().IndexOf("<EOF>") != -1)
                {
                    break;
                }
            } while (bytes !=0);

            return messageData.ToString();
        }
         static void DisplaySecurityLevel(SslStream stream)
         {
            Console.WriteLine("Cipher: {0} strength {1}", stream.CipherAlgorithm, stream.CipherStrength);
            Console.WriteLine("Hash: {0} strength {1}", stream.HashAlgorithm, stream.HashStrength);
            Console.WriteLine("Key exchange: {0} strength {1}", stream.KeyExchangeAlgorithm, stream.KeyExchangeStrength);
            Console.WriteLine("Protocol: {0}", stream.SslProtocol);
         }
         static void DisplaySecurityServices(SslStream stream)
         {
            Console.WriteLine("Is authenticated: {0} as server? {1}", stream.IsAuthenticated, stream.IsServer);
            Console.WriteLine("IsSigned: {0}", stream.IsSigned);
            Console.WriteLine("Is Encrypted: {0}", stream.IsEncrypted);
            Console.WriteLine("Is mutually authenticated: {0}", stream.IsMutuallyAuthenticated);
         }
         static void DisplayStreamProperties(SslStream stream)
         {
            Console.WriteLine("Can read: {0}, write {1}", stream.CanRead, stream.CanWrite);
            Console.WriteLine("Can timeout: {0}", stream.CanTimeout);
         }
        static void DisplayCertificateInformation(SslStream stream)
        {
            Console.WriteLine("Certificate revocation list checked: {0}", stream.CheckCertRevocationStatus);

            X509Certificate localCertificate = stream.LocalCertificate;
            if (stream.LocalCertificate != null)
            {
                Console.WriteLine("Local cert was issued to {0} and is valid from {1} until {2}.",
                    localCertificate.Subject,
                    localCertificate.GetEffectiveDateString(),
                    localCertificate.GetExpirationDateString());
             } else
            {
                Console.WriteLine("Local certificate is null.");
            }
            // Display the properties of the client's certificate.
            X509Certificate remoteCertificate = stream.RemoteCertificate;
            if (stream.RemoteCertificate != null)
            {
            Console.WriteLine("Remote cert was issued to {0} and is valid from {1} until {2}.",
                remoteCertificate.Subject,
                remoteCertificate.GetEffectiveDateString(),
                remoteCertificate.GetExpirationDateString());
            } else
            {
                Console.WriteLine("Remote certificate is null.");
            }
        }
        private static void DisplayUsage()
        {
            Console.WriteLine("To start the server specify:");
            Console.WriteLine("serverSync certificateFile.cer");
            Environment.Exit(1);
        }
        public static int Main(string[] args)
        {
            string certificate = null;
            if (args == null ||args.Length < 1 )
            {
                DisplayUsage();
            }
            certificate = args[0];
            SslTcpServer.RunServer (certificate);
            return 0;
        }
    }
}

**** TcpClient
The following code example demonstrates creating a TcpClient that uses the SslStream class to communicate with a server.
using System;
using System.Collections;
using System.Net;
using System.Net.Security;
using System.Net.Sockets;
using System.Security.Authentication;
using System.Text;
using System.Security.Cryptography.X509Certificates;
using System.IO;

namespace Examples.System.Net
{
    public class SslTcpClient
    {
        private static Hashtable certificateErrors = new Hashtable();

        // The following method is invoked by the RemoteCertificateValidationDelegate.
        public static bool ValidateServerCertificate(
              object sender,
              X509Certificate certificate,
              X509Chain chain,
              SslPolicyErrors sslPolicyErrors)
        {
           if (sslPolicyErrors == SslPolicyErrors.None)
                return true;

            Console.WriteLine("Certificate error: {0}", sslPolicyErrors);

            // Do not allow this client to communicate with unauthenticated servers.
            return false;
        }
        public static void RunClient(string machineName, string serverName)
        {
            // Create a TCP/IP client socket.
            // machineName is the host running the server application.
            TcpClient client = new TcpClient(machineName,5000);
            Console.WriteLine("Client connected.");
            // Create an SSL stream that will close the client's stream.
            SslStream sslStream = new SslStream(
                client.GetStream(),
                false,
                new RemoteCertificateValidationCallback (ValidateServerCertificate),
                null
                );
            // The server name must match the name on the server certificate.
            try
            {
                sslStream.AuthenticateAsClient(serverName);
            }
            catch (AuthenticationException e)
            {
                Console.WriteLine("Exception: {0}", e.Message);
                if (e.InnerException != null)
                {
                    Console.WriteLine("Inner exception: {0}", e.InnerException.Message);
                }
                Console.WriteLine ("Authentication failed - closing the connection.");
                client.Close();
                return;
            }
            // Encode a test message into a byte array.
            // Signal the end of the message using the "<EOF>".
            byte[] messsage = Encoding.UTF8.GetBytes("Hello from the client.<EOF>");
            // Send hello message to the server.
            sslStream.Write(messsage);
            sslStream.Flush();
            // Read message from the server.
            string serverMessage = ReadMessage(sslStream);
            Console.WriteLine("Server says: {0}", serverMessage);
            // Close the client connection.
            client.Close();
            Console.WriteLine("Client closed.");
        }
        static string ReadMessage(SslStream sslStream)
        {
            // Read the  message sent by the server.
            // The end of the message is signaled using the
            // "<EOF>" marker.
            byte [] buffer = new byte[2048];
            StringBuilder messageData = new StringBuilder();
            int bytes = -1;
            do
            {
                bytes = sslStream.Read(buffer, 0, buffer.Length);

                // Use Decoder class to convert from bytes to UTF8
                // in case a character spans two buffers.
                Decoder decoder = Encoding.UTF8.GetDecoder();
                char[] chars = new char[decoder.GetCharCount(buffer,0,bytes)];
                decoder.GetChars(buffer, 0, bytes, chars,0);
                messageData.Append (chars);
                // Check for EOF.
                if (messageData.ToString().IndexOf("<EOF>") != -1)
                {
                    break;
                }
            } while (bytes != 0);

            return messageData.ToString();
        }
        private static void DisplayUsage()
        {
            Console.WriteLine("To start the client specify:");
            Console.WriteLine("clientSync machineName [serverName]");
            Environment.Exit(1);
        }
        public static int Main(string[] args)
        {
            string serverCertificateName = null;
            string machineName = null;
            if (args == null ||args.Length <1 )
            {
                DisplayUsage();
            }
            // User can specify the machine name and server name.
            // Server name must match the name on the server's certificate.
            machineName = args[0];
            if (args.Length <2 )
            {
                serverCertificateName = machineName;
            }
            else
            {
                serverCertificateName = args[1];
            }
            SslTcpClient.RunClient (machineName, serverCertificateName);
            return 0;
        }
    }
}

Remarks
SSL protocols help to provide confidentiality and integrity checking for messages transmitted using an SslStream. 
An SSL connection, such as that provided by SslStream, should be used when communicating sensitive information between a client and a server. 
Using an SslStream helps to prevent anyone from reading and tampering with information while it is in transit on the network.

An SslStream instance transmits data using a stream that you supply when creating the SslStream. 
When you supply this underlying stream, you have the option to specify whether closing the SslStream also closes the underlying stream. 
Typically, the SslStream class is used with the TcpClient and TcpListener classes. 
The GetStream method provides a NetworkStream suitable for use with the SslStream class.

After creating an SslStream, the server and optionally, the client must be authenticated. 
The server must provide an X509 certificate that establishes proof of its identity and can request that the client also do so. 
Authentication must be performed before transmitting information using an SslStream. 
Clients initiate authentication using the synchronous AuthenticateAsClient methods, which block until the authentication completes, or the asynchronous BeginAuthenticateAsClient methods, which do not block waiting for the authentication to complete. 
Servers initiate authentication using the synchronous AuthenticateAsServer or asynchronous BeginAuthenticateAsServer methods. 
Both client and server must initiate the authentication.

The authentication is handled by the Security Support Provider (SSPI) channel provider. 
The client is given an opportunity to control validation of the server's certificate by specifying a RemoteCertificateValidationCallback delegate when creating an SslStream. 
The server can also control validation by supplying a RemoteCertificateValidationCallback delegate. 
The method referenced by the delegate includes the remote party's certificate and any errors SSPI encountered while validating the certificate. 
Note that if the server specifies a delegate, the delegate's method is invoked regardless of whether the server requested client authentication. 
If the server did not request client authentication, the server's delegate method receives a null certificate and an empty array of certificate errors.

If the server requires client authentication, the client must specify one or more certificates for authentication. 
If the client has more than one certificate, the client can provide a LocalCertificateSelectionCallback delegate to select the correct certificate for the server. 
The client's certificates must be located in the current user's "My" certificate store. 
Client authentication via certificates is not supported for the Ssl2 (SSL version 2) protocol.

If the authentication fails, you receive a AuthenticationException, and the SslStream is no longer useable. 
You should close this object and remove all references to it so that it can be collected by the garbage collector.

When the authentication process, also known as the SSL handshake, succeeds, the identity of the server (and optionally, the client) is established and the SslStream can be used by the client and server to exchange messages. 
Before sending or receiving information, the client and server should check the security services and levels provided by the SslStream to determine whether the protocol, algorithms, and strengths selected meet their requirements for integrity and confidentiality. 
If the current settings are not sufficient, the stream should be closed. You can check the security services provided by the SslStream using the IsEncrypted and IsSigned properties. 
The following table shows the elements that report the cryptographic settings used for authentication, encryption and data signing.

Element	                                                                           Members
---------------------------------------------------------------------------------- ------------------------------------
The security protocol used to authenticate the server and, optionally, the client. The SslProtocol property and the associated SslProtocols enumeration.
The key exchange algorithm.	                                                       The KeyExchangeAlgorithm property and the associated ExchangeAlgorithmType enumeration.
The message integrity algorithm.	                                               The HashAlgorithm property and the associated HashAlgorithmType enumeration.
The message confidentiality algorithm.	                                           The CipherAlgorithm property and the associated CipherAlgorithmType enumeration.
The strengths of the selected algorithms.	                                       The KeyExchangeStrength, HashStrength, and CipherStrength properties.
---------------------------------------------------------------------------------- ------------------------------------


After a successful authentication, you can send data using the synchronous Write or asynchronous BeginWrite methods. 
You can receive data using the synchronous Read or asynchronous BeginRead methods.

If you specified to the SslStream that the underlying stream should be left open, you are responsible for closing that stream when you are done using it.


Note
If the application that creates the SslStream object runs with the credentials of a Normal user, 
the application will not be able to access certificates installed in the local machine store unless permission has been explicitly given to the user to do so.

SslStream assumes that a timeout along with any other IOException when one is thrown from the inner stream will be treated as fatal by its caller. 
Reusing a SslStream instance after a timeout will return garbage. 
An application should Close the SslStream and throw an exception in these cases.

The .NET Framework 4.6 includes a new security feature that blocks insecure cipher and hashing algorithms for connections. 
Applications using TLS/SSL through APIs such as HttpClient, HttpWebRequest, FTPClient, SmtpClient, SslStream, etc. and targeting .NET Framework 4.6 get the more-secure behavior by default.

Developers may want to opt out of this behavior in order to maintain interoperability with their existing SSL3 services OR TLS w/ RC4 services. 
This article explains how to modify your code so that the new behavior is disabled.

The .NET Framework 4.7 adds new overloads for the methods that authenticate SslStreams that do not specify a TLS version, but instead use the TLS version defined as the system default in SCHANNEL. 
Use these methods in your app as a way to be able to later modify the defaults as TLS version best practice changes over time, without the need to rebuild and redeploy your app.

Also see Transport Layer Security (TLS) best practices with the .NET Framework. https://learn.microsoft.com/en-us/dotnet/framework/network-programming/tls



#### 以下確定的移到上面
2021-11-27

ref:
https://docs.microsoft.com/en-us/dotnet/api/system.net.security.sslstream?redirectedfrom=MSDN&view=net-6.0


Definition
Namespace: System.Net.Security
Assembly: System.Net.Security.dll

Provides a stream used for client-server communication that uses the Secure Socket Layer (SSL) security protocol to authenticate the server and optionally the client.

public class SslStream : System.Net.Security.AuthenticatedStream

Inheritance
Object.MarshalByRefObject.Stream.AuthenticatedStream.SslStream


Examples
The following code example demonstrates creating an TcpListener that uses the SslStream class to communicate with clients.

using System;
using System.Collections;
using System.Net;
using System.Net.Sockets;
using System.Net.Security;
using System.Security.Authentication;
using System.Text;
using System.Security.Cryptography.X509Certificates;
using System.IO;

namespace Examples.System.Net
{
    public sealed class SslTcpServer
    {
        static X509Certificate serverCertificate = null;
        // The certificate parameter specifies the name of the file
        // containing the machine certificate.
        public static void RunServer(string certificate)
        {
            serverCertificate = X509Certificate.CreateFromCertFile(certificate);
            // Create a TCP/IP (IPv4) socket and listen for incoming connections.
            TcpListener listener = new TcpListener(IPAddress.Any, 8080);
            listener.Start();
            while (true)
            {
                Console.WriteLine("Waiting for a client to connect...");
                // Application blocks while waiting for an incoming connection.
                // Type CNTL-C to terminate the server.
                TcpClient client = listener.AcceptTcpClient();
                ProcessClient(client);
            }
        }
        static void ProcessClient (TcpClient client)
        {
            // A client has connected. Create the
            // SslStream using the client's network stream.
            SslStream sslStream = new SslStream(
                client.GetStream(), false);
            // Authenticate the server but don't require the client to authenticate.
            try
            {
                sslStream.AuthenticateAsServer(serverCertificate, clientCertificateRequired: false, checkCertificateRevocation: true);

                // Display the properties and settings for the authenticated stream.
                DisplaySecurityLevel(sslStream);
                DisplaySecurityServices(sslStream);
                DisplayCertificateInformation(sslStream);
                DisplayStreamProperties(sslStream);

                // Set timeouts for the read and write to 5 seconds.
                sslStream.ReadTimeout = 5000;
                sslStream.WriteTimeout = 5000;
                // Read a message from the client.
                Console.WriteLine("Waiting for client message...");
                string messageData = ReadMessage(sslStream);
                Console.WriteLine("Received: {0}", messageData);

                // Write a message to the client.
                byte[] message = Encoding.UTF8.GetBytes("Hello from the server.<EOF>");
                Console.WriteLine("Sending hello message.");
                sslStream.Write(message);
            }
            catch (AuthenticationException e)
            {
                Console.WriteLine("Exception: {0}", e.Message);
                if (e.InnerException != null)
                {
                    Console.WriteLine("Inner exception: {0}", e.InnerException.Message);
                }
                Console.WriteLine ("Authentication failed - closing the connection.");
                sslStream.Close();
                client.Close();
                return;
            }
            finally
            {
                // The client stream will be closed with the sslStream
                // because we specified this behavior when creating
                // the sslStream.
                sslStream.Close();
                client.Close();
            }
        }
        static string ReadMessage(SslStream sslStream)
        {
            // Read the  message sent by the client.
            // The client signals the end of the message using the
            // "<EOF>" marker.
            byte [] buffer = new byte[2048];
            StringBuilder messageData = new StringBuilder();
            int bytes = -1;
            do
            {
                // Read the client's test message.
                bytes = sslStream.Read(buffer, 0, buffer.Length);

                // Use Decoder class to convert from bytes to UTF8
                // in case a character spans two buffers.
                Decoder decoder = Encoding.UTF8.GetDecoder();
                char[] chars = new char[decoder.GetCharCount(buffer,0,bytes)];
                decoder.GetChars(buffer, 0, bytes, chars,0);
                messageData.Append (chars);
                // Check for EOF or an empty message.
                if (messageData.ToString().IndexOf("<EOF>") != -1)
                {
                    break;
                }
            } while (bytes !=0);

            return messageData.ToString();
        }
         static void DisplaySecurityLevel(SslStream stream)
         {
            Console.WriteLine("Cipher: {0} strength {1}", stream.CipherAlgorithm, stream.CipherStrength);
            Console.WriteLine("Hash: {0} strength {1}", stream.HashAlgorithm, stream.HashStrength);
            Console.WriteLine("Key exchange: {0} strength {1}", stream.KeyExchangeAlgorithm, stream.KeyExchangeStrength);
            Console.WriteLine("Protocol: {0}", stream.SslProtocol);
         }
         static void DisplaySecurityServices(SslStream stream)
         {
            Console.WriteLine("Is authenticated: {0} as server? {1}", stream.IsAuthenticated, stream.IsServer);
            Console.WriteLine("IsSigned: {0}", stream.IsSigned);
            Console.WriteLine("Is Encrypted: {0}", stream.IsEncrypted);
         }
         static void DisplayStreamProperties(SslStream stream)
         {
            Console.WriteLine("Can read: {0}, write {1}", stream.CanRead, stream.CanWrite);
            Console.WriteLine("Can timeout: {0}", stream.CanTimeout);
         }
        static void DisplayCertificateInformation(SslStream stream)
        {
            Console.WriteLine("Certificate revocation list checked: {0}", stream.CheckCertRevocationStatus);

            X509Certificate localCertificate = stream.LocalCertificate;
            if (stream.LocalCertificate != null)
            {
                Console.WriteLine("Local cert was issued to {0} and is valid from {1} until {2}.",
                    localCertificate.Subject,
                    localCertificate.GetEffectiveDateString(),
                    localCertificate.GetExpirationDateString());
             } else
            {
                Console.WriteLine("Local certificate is null.");
            }
            // Display the properties of the client's certificate.
            X509Certificate remoteCertificate = stream.RemoteCertificate;
            if (stream.RemoteCertificate != null)
            {
            Console.WriteLine("Remote cert was issued to {0} and is valid from {1} until {2}.",
                remoteCertificate.Subject,
                remoteCertificate.GetEffectiveDateString(),
                remoteCertificate.GetExpirationDateString());
            } else
            {
                Console.WriteLine("Remote certificate is null.");
            }
        }
        private static void DisplayUsage()
        {
            Console.WriteLine("To start the server specify:");
            Console.WriteLine("serverSync certificateFile.cer");
            Environment.Exit(1);
        }
        public static int Main(string[] args)
        {
            string certificate = null;
            if (args == null ||args.Length < 1 )
            {
                DisplayUsage();
            }
            certificate = args[0];
            SslTcpServer.RunServer (certificate);
            return 0;
        }
    }
}


The following code example demonstrates creating a TcpClient that uses the SslStream class to communicate with a server.

using System;
using System.Collections;
using System.Net;
using System.Net.Security;
using System.Net.Sockets;
using System.Security.Authentication;
using System.Text;
using System.Security.Cryptography.X509Certificates;
using System.IO;

namespace Examples.System.Net
{
    public class SslTcpClient
    {
        private static Hashtable certificateErrors = new Hashtable();

        // The following method is invoked by the RemoteCertificateValidationDelegate.
        public static bool ValidateServerCertificate(
              object sender,
              X509Certificate certificate,
              X509Chain chain,
              SslPolicyErrors sslPolicyErrors)
        {
           if (sslPolicyErrors == SslPolicyErrors.None)
                return true;

            Console.WriteLine("Certificate error: {0}", sslPolicyErrors);

            // Do not allow this client to communicate with unauthenticated servers.
            return false;
        }
        public static void RunClient(string machineName, string serverName)
        {
            // Create a TCP/IP client socket.
            // machineName is the host running the server application.
            TcpClient client = new TcpClient(machineName,443);
            Console.WriteLine("Client connected.");
            // Create an SSL stream that will close the client's stream.
            SslStream sslStream = new SslStream(
                client.GetStream(),
                false,
                new RemoteCertificateValidationCallback (ValidateServerCertificate),
                null
                );
            // The server name must match the name on the server certificate.
            try
            {
                sslStream.AuthenticateAsClient(serverName);
            }
            catch (AuthenticationException e)
            {
                Console.WriteLine("Exception: {0}", e.Message);
                if (e.InnerException != null)
                {
                    Console.WriteLine("Inner exception: {0}", e.InnerException.Message);
                }
                Console.WriteLine ("Authentication failed - closing the connection.");
                client.Close();
                return;
            }
            // Encode a test message into a byte array.
            // Signal the end of the message using the "<EOF>".
            byte[] messsage = Encoding.UTF8.GetBytes("Hello from the client.<EOF>");
            // Send hello message to the server.
            sslStream.Write(messsage);
            sslStream.Flush();
            // Read message from the server.
            string serverMessage = ReadMessage(sslStream);
            Console.WriteLine("Server says: {0}", serverMessage);
            // Close the client connection.
            client.Close();
            Console.WriteLine("Client closed.");
        }
        static string ReadMessage(SslStream sslStream)
        {
            // Read the  message sent by the server.
            // The end of the message is signaled using the
            // "<EOF>" marker.
            byte [] buffer = new byte[2048];
            StringBuilder messageData = new StringBuilder();
            int bytes = -1;
            do
            {
                bytes = sslStream.Read(buffer, 0, buffer.Length);

                // Use Decoder class to convert from bytes to UTF8
                // in case a character spans two buffers.
                Decoder decoder = Encoding.UTF8.GetDecoder();
                char[] chars = new char[decoder.GetCharCount(buffer,0,bytes)];
                decoder.GetChars(buffer, 0, bytes, chars,0);
                messageData.Append (chars);
                // Check for EOF.
                if (messageData.ToString().IndexOf("<EOF>") != -1)
                {
                    break;
                }
            } while (bytes != 0);

            return messageData.ToString();
        }
        private static void DisplayUsage()
        {
            Console.WriteLine("To start the client specify:");
            Console.WriteLine("clientSync machineName [serverName]");
            Environment.Exit(1);
        }
        public static int Main(string[] args)
        {
            string serverCertificateName = null;
            string machineName = null;
            if (args == null ||args.Length <1 )
            {
                DisplayUsage();
            }
            // User can specify the machine name and server name.
            // Server name must match the name on the server's certificate.
            machineName = args[0];
            if (args.Length <2 )
            {
                serverCertificateName = machineName;
            }
            else
            {
                serverCertificateName = args[1];
            }
            SslTcpClient.RunClient (machineName, serverCertificateName);
            return 0;
        }
    }
}

Remarks
SSL protocols help to provide confidentiality and integrity checking for messages transmitted using an SslStream. An SSL connection, such as that provided by SslStream, should be used when communicating sensitive information between a client and a server. Using an SslStream helps to prevent anyone from reading and tampering with information while it is in transit on the network.

An SslStream instance transmits data using a stream that you supply when creating the SslStream. When you supply this underlying stream, you have the option to specify whether closing the SslStream also closes the underlying stream. Typically, the SslStream class is used with the TcpClient and TcpListener classes. The GetStream method provides a NetworkStream suitable for use with the SslStream class.

After creating an SslStream, the server and optionally, the client must be authenticated. The server must provide an X509 certificate that establishes proof of its identity and can request that the client also do so. Authentication must be performed before transmitting information using an SslStream. Clients initiate authentication using the synchronous AuthenticateAsClient methods, which block until the authentication completes, or the asynchronous BeginAuthenticateAsClient methods, which do not block waiting for the authentication to complete. Servers initiate authentication using the synchronous AuthenticateAsServer or asynchronous BeginAuthenticateAsServer methods. Both client and server must initiate the authentication.

The authentication is handled by the Security Support Provider (SSPI) channel provider. The client is given an opportunity to control validation of the server's certificate by specifying a RemoteCertificateValidationCallback delegate when creating an SslStream. The server can also control validation by supplying a RemoteCertificateValidationCallback delegate. The method referenced by the delegate includes the remote party's certificate and any errors SSPI encountered while validating the certificate. Note that if the server specifies a delegate, the delegate's method is invoked regardless of whether the server requested client authentication. If the server did not request client authentication, the server's delegate method receives a null certificate and an empty array of certificate errors.

If the server requires client authentication, the client must specify one or more certificates for authentication. If the client has more than one certificate, the client can provide a LocalCertificateSelectionCallback delegate to select the correct certificate for the server. The client's certificates must be located in the current user's "My" certificate store. Client authentication via certificates is not supported for the Ssl2 (SSL version 2) protocol.

If the authentication fails, you receive a AuthenticationException, and the SslStream is no longer useable. You should close this object and remove all references to it so that it can be collected by the garbage collector.

When the authentication process, also known as the SSL handshake, succeeds, the identity of the server (and optionally, the client) is established and the SslStream can be used by the client and server to exchange messages. Before sending or receiving information, the client and server should check the security services and levels provided by the SslStream to determine whether the protocol, algorithms, and strengths selected meet their requirements for integrity and confidentiality. If the current settings are not sufficient, the stream should be closed. You can check the security services provided by the SslStream using the IsEncrypted and IsSigned properties. The following table shows the elements that report the cryptographic settings used for authentication, encryption and data signing.

TABLE 1
Element	Members

1. The security protocol used to authenticate the server and, optionally, the client.	The SslProtocol property and the associated SslProtocols enumeration.

2. The key exchange algorithm.	The KeyExchangeAlgorithm property and the associated ExchangeAlgorithmType enumeration.

3. The message integrity algorithm.	The HashAlgorithm property and the associated HashAlgorithmType enumeration.

4. The message confidentiality algorithm.	The CipherAlgorithm property and the associated CipherAlgorithmType enumeration.

5. The strengths of the selected algorithms.	The KeyExchangeStrength, HashStrength, and CipherStrength properties.



After a successful authentication, you can send data using the synchronous Write or asynchronous BeginWrite methods. You can receive data using the synchronous Read or asynchronous BeginRead methods.

If you specified to the SslStream that the underlying stream should be left open, you are responsible for closing that stream when you are done using it.

 Note

If the application that creates the SslStream object runs with the credentials of a Normal user, the application will not be able to access certificates installed in the local machine store unless permission has been explicitly given to the user to do so.

SslStream assumes that a timeout along with any other IOException when one is thrown from the inner stream will be treated as fatal by its caller. Reusing a SslStream instance after a timeout will return garbage. An application should Close the SslStream and throw an exception in these cases.

The .NET Framework 4.6 includes a new security feature that blocks insecure cipher and hashing algorithms for connections. Applications using TLS/SSL through APIs such as HttpClient, HttpWebRequest, FTPClient, SmtpClient, SslStream, etc. and targeting .NET Framework 4.6 get the more-secure behavior by default.

Developers may want to opt out of this behavior in order to maintain interoperability with their existing SSL3 services OR TLS w/ RC4 services. This article explains how to modify your code so that the new behavior is disabled.

The .NET Framework 4.7 adds new overloads for the methods that authenticate SslStreams that do not specify a TLS version, but instead use the TLS version defined as the system default in SCHANNEL. Use these methods in your app as a way to be able to later modify the defaults as TLS version best practice changes over time, without the need to rebuild and redeploy your app.